This invention relates to an improved method for manufacturing power switching devices, such as gate turnoff thyristors (GTO), SCRs, light activated SCRs and power transistors.
Generally, thyristors have PNPN structures including a P-type anode layer, an N-type base layer, a P-type base layer and a N-type cathode layer.
For such thyristors, it is desired that minority carrier lifetimes in their P-type base layers are sufficiently high and those in their N-type base layers are sufficiently low, to allow anode currents of the thyristors to increase with an increase in the carrier lifetimes in the P-type base layers, and to allow the switching characteristics to improve with a decrease in the carrier lifetimes in the N-type base layers. Thereby, preferable thyristors characteristics are obtained.
A method for manufacturing such a desirable thyristor is disclosed in Japanese Journal of Applied Physics, Vol. 17, Supp. 17-1, pp. 275-281, 1978, "High Power Gate Turn-Off Thyristors." According to this method, a PNP structure is made at first by diffusing P-type impurities into an N-type substrate from both its surfaces. Next, a film containing phosphorus as an N-type impurity is deposited on one side-surface of the diffused PNP wafer. Thereafter, a PNPN structure is formed by a drive-in process.
In the above method, the film containing phosphorus should be essentially removed before the phosphorus drive-in, although this is not shown apparently in the publication.
The reason is that if the film containing phosphorus exists on the wafer surface, the phosphorus content in the N-type cathode layer will be extremely difficult to control during the phosphorus drive-in.
At this step, if too much phosphorus is diffused into the P-type base layer in error, breakdown voltage of the P-base and N-emitter junction decreases due to an increase in the impurity gradient.
According to the above publication, after the step of the phosphorus drive-in, gold is diffused as carrier life-time killer atoms into the wafer, specifically into the N-type base layer. Through the gold diffusion, minority carrier lifetime of the N-type base decreases. This process contributes to improvement of the switching characteristics of the thyristor.
It is also possible to expect that the phosphorus film formation on the substrate will improve minority carrier lifetime of the P-type base. The reason being that a phosphorus glass has a gettering effect on metal contaminations, such as gold and copper in silicon, as is disclosed in the Journal of the Electrochemical Society, June, 1963, pp. 533-537, "Gettering of Metallic Impurities from Planar Silicon Diodes" and in the Solid-State Electronics Pergamon Press, Vol. 11, pp. 1055-1061, 1968, "The Gettering of Gold and Copper from Silicon."
Especially, in the latter publication, hole lifetime values after phosphorus gettering are shown.
But unexpectedly, according to the actually manufactured thyristors obtained from the above method, carrier lifetimes in the P-type bases are mostly unchanged and low.
As a result, thyristors having high current, high surge current, low on-state voltage and high off-state voltage thus far have not been obtained.